1. Field of the Invention
The present invention relates to a picture processing apparatus, a picture processing method, an information processing apparatus, a recording medium, and a program. In particular, the present invention relates to a picture processing apparatus, a picture processing method, an information processing apparatus, a recording medium, and a program which are preferably used when corresponding data can be re-encoded by using information about a past encoding process performed thereon.
2. Description of the Related Art
In a system for transmitting a moving-picture signal to a remote location, such as a video conference system and a video telephone system, the moving-picture signal is compressed and encoded by using a line correlation or an interframe correlation of the picture signal, so as to efficiently use a transmission line.
The picture signal is compressed and encoded so that a bit stream to be generated has a predetermined bit rate. However, the bit rate of the bit stream may have to be changed according to the condition of a transmission line in an actual operation.
For example, when a transmitted picture signal is edited by a broadcasting station, the edit process is performed in units of seconds. Therefore, picture information of each frame should be independent from that of another frame. Accordingly, a long GOP (group of pictures, which is a group of frames having correlated information) including many frames so that the picture quality is not deteriorated even when transferred at a low bit rate (for example, 3 to 9 Mbps) and a short GOP including fewer frames transferred at a high bit rate (18 to 50 Mbps) need to be mutually converted.
For example, a system for encoding uncompressed data into MPEG long GOP stream data and then editing the frames thereof will be described with reference to FIG. 1.
An SDI (serial digital interface)-ASI (asynchronous serial interface) converter 1 encodes an input SDI picture into an MPEG long GOP (ASI stream) and outputs the encoded MPEG long GOP stream data. The SDI is a transmission scheme for uncompressed digital video/audio based on a point-to-point transmission, and is standardized by ANSI (American National Standards Institute)/SMPTE (Society of Motion Picture and Television Engineers) 259M.
An ASI-SDTI CP (serial data transport interface contents package) converter 2 decodes the supplied MPEG long GOP stream data by a decoding unit 21, encodes the entire stream data into intraframes by an encoding unit 22, and then outputs the encoded stream data composed of intraframes (SDTI CP stream) to a frame editor 3 of an SDTI CP interface. The SDTI CP is a global standard of a transmission scheme for transmitting MPEG data in real time (synchronous transfer), which is standardized as SMPTE 326M by the promotion of the Pro-MPEG forum.
The stream data which has been frame-edited by the frame editor 3 is supplied to an SDTI CP-ASI converter 4. The SDTI CP-ASI converter 4 decodes the supplied stream data composed of intraframes by a decoding unit 31, encodes it into an MPEG long GOP by an encoding unit 32, and then outputs the encoded MPEG long GOP stream data (ASI stream).
Next, a system for encoding an input picture into an MPEG long GOP at a high bit rate, decoding it, and then re-encoding it into a low-bit-rate MPEG long GOP will be described with reference to FIG. 2.
A long GOP encoder 51 decodes an input SDI picture by a decoding unit 61, encodes it into a high-bit-rate MPEG long GOP by an encoding unit 62, and then outputs the encoded MPEG long GOP stream (ASI stream) data. A long GOP encoder 52 decodes the supplied high-bit-rate MPEG long GOP by a decoding unit 71, encodes it into a low-bit-rate MPEG long GOP by an encoding unit 72, and then outputs the encoded low-bit-rate MPEG long GOP stream (ASI stream) data.
In such a case where picture information is repeatedly encoded and decoded, the picture information deteriorates if encoding parameters are changed at each encoding process. In order to prevent the deterioration of the picture information, a technique for suppressing picture deterioration caused by re-encoding is proposed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2000-059788). In this technique, encoding history information inserted into a user data area in a picture layer of a bit stream is used.
FIG. 3 shows a system for encoding uncompressed data into MPEG long GOP stream data and then editing the frames thereof. In this system, encoding history information is used. In FIG. 3, parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and the corresponding description will be omitted.
An ASI-SDTI CP converter 101 receives an MPEG long GOP (ASI stream) which has been generated by an encoding process performed by the SDI-ASI converter 1, which is the same as that in FIG. 1.
The MPEG long GOP is composed of three types of pictures (I-picture, P-picture, and B-picture), each having a different encoding feature. Therefore, video data generated by decoding the MPEG long GOP includes frames, each having the feature of I-picture, P-picture, or B-picture. When the video data is re-encoded into an MPEG long GOP, the picture quality may be deteriorated if each of the I-picture, P-picture, and B-picture in the video data is encoded with another picture type. For example, if video data which was a B-picture before decoding (the B-picture is more likely to have distortion than an I-picture and a P-picture) is encoded as an I-picture, neighboring pictures are predictively encoded by using the I-picture having much distortion as a reference picture. Accordingly, the picture quality deteriorates.
In order to prevent the deterioration of the picture quality caused by re-encoding, the ASI-SDTI CP converter 101 decodes the supplied MPEG long GOP stream data by a decoding unit 111, and then encodes the entire stream data into intraframes by an encoding unit 112. At this time, the ASI-SDTI CP converter 101 adds parameters such as a picture type and a quantization value used in a past encoding process, that is, in the encoding process performed by the SDI-ASI converter 1, as history information (history data) of SMPTE 328M to the SDTI CP stream composed of intraframes, which is then supplied to the frame editor 3.
The stream data is frame-edited by the frame editor 3 and is then supplied to an SDTI CP-ASI converter 102. The SDTI CP-ASI converter 102 decodes the supplied stream data composed of intraframes added with the history information by a decoding unit 121. Then, an encoding unit 122 re-encodes the stream data into a long GOP by using necessary parameters, such as a picture type and a quantization value, which are included in the decoded history information, and then outputs the long GOP.
In the system shown in FIG. 2, an input picture is encoded into an MPEG long GOP at a high bit rate, and the MPEG long GOP is decoded and is then re-encoded into a low-bit-rate MPEG long GOP. FIG. 4 shows a case where the picture quality is not deteriorated due to a re-encoding process in the system shown in FIG. 2. In FIG. 4, parts corresponding to those in FIG. 2 are denoted by the same reference numerals, and the corresponding description will be omitted.
A long GOP encoder 131 receives MPEG long GOP stream (ASI stream) data, which has been encoded by the long GOP encoder 51, and decodes the high-bit-rate MPEG long GOP by a decoding unit 141. At this time, the long GOP encoder 131 obtains necessary encoding parameters and supplies them to an encoding unit 142 together with the decoded video data. The encoding unit 142 encodes the video data into a low-bit-rate MPEG long GOP by using the supplied encoding parameters, and outputs the encoded low-bit-rate MPEG long GOP stream (ASI stream) data.
As described above, deterioration of the picture quality can be prevented by encoding video data by reusing information about past encoding, such as history information or encoding parameters (picture type, motion vector, and quantization value in the past encoding). However, as in the system shown in FIG. 3, when a process of encoding entire data into intraframes is performed between the past encoding process, in which data is encoded into an MPEG long-GOP, and the encoding process by reusing parameters, a VBV buffer failure may be caused at re-encoding.
The VBV buffer failure at re-encoding is caused in the following case. That is, a picture encoded in a past encoding process is distorted due to an insufficiently high bit rate of an intermediate encoding process (process of encoding entire data into intraframes). If a quantization value which was used for encoding an undistorted picture is used for re-encoding the distorted picture, the VBV buffer failure occurs.
More specifically, when an SDI static picture is encoded into an MPEG long GOP by the SDI-ASI converter 1, a large amount of code is generated in an I-picture, whereas a small amount of code is generated in P- and B-pictures, as shown in FIG. 5, which illustrates the track of the VBV buffer. This is because the I-picture forms the most part of the input picture and that the following P- and B-pictures are encoded with a minor differential thereof. When all of these pictures are encoded into intraframes by the ASI-SDTI CP converter 101, the encoding can be performed faithfully to the picture which has been previously encoded if a sufficiently high bit rate is used. However, if the bit rate is low, the previously-encoded picture is encoded so as to have distortion.
Assume that the distorted picture is re-encoded by using a quantization value or the like used in the past encoding process performed by the SDI-ASI converter 1 in the SDTI CP-ASI converter 102. In this case, as shown in FIG. 6, the amount of generated code is small in the picture which was an I-picture because data mainly composed of high-frequency components has been reduced. On the other hand, in the pictures which were P- and B-pictures, data having a differential increased by distortion is encoded by using a quantization value which was used in the SDI-ASI converter 1, which has encoded small-differential data. Accordingly, a large amount of code is generated in the re-encoding process performed by the SDTI CP-ASI converter 102, so that a VBV buffer failure is caused.